Typically, an integrated optical circuit has at least one coupler from which there is obtained signals having a mutual phase shift. U.S. Pat. No. 4.938.595 issued Jul. 3, 1990 uses a 2.times.3 coupler which produces three output signals having a mutual phase shift of 120.degree. . When a 2.times.4 coupler is used, four signals are produced having a mutual phase shift of 90.degree..
However, if a 2.times.2 coupler is used, as described in U.S. Pat. No. 4,759,627 issued Jul. 26, 1988, two signals are produced having a mutual phase shift of 180.degree..
For many applications, a photoelectric position measuring device requires the detection of measuring direction The prior art, however, has several disadvantages. If a 2.times.2 coupler is used, the measuring direction cannot be detected since the two output signals have a mutual phase shift of 180.degree.. If a 2.times.3 or a 2.times.4 coupler is used, the measuring direction can be detected, however, couplers having more than two outputs have certain disadvantages associated therewith. First, the coupling properties are changed, for example, because of output fluctuations, and thus the degree of modulation and the phase relation between the signals at the output are affected. Second, relative changes of the amplitudes of the two signals at the two inputs of the waveguide coupler generate changes in the degree of modulation and the phase relation between the signals at the outputs.
Accordingly, it is a primary object of the present invention to provide an apparatus and method to generate in a simple manner at least two signals with a mutual phase shift other than 180.degree. using at least one waveguide coupler.
Another object of the present invention is to provide a 2.times.2 coupler for use in position measuring apparatus and method which is able to determine the measuring direction by providing at least one damping element or phase shifting element in at least one 2.times.2 coupler to generate by simple means two signals at its outputs with a mutual phase shift other than 180.degree..